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Additive Manufacturing 1–4 (2014) 110–118

Light curing strategies for lithography-based additive manufacturing ofcustomized ceramics

Gerald Mitteramskogler a,∗, Robert Gmeiner a, Ruth Felzmann a, Simon Gruber b,Christoph Hofstetter a, Jürgen Stampfl a, Jörg Ebert c, Wolfgang Wachter c, Jürgen Laubersheimer c

Christian Doppler Laboratory for Photopolymers in Digital and Restorative Dentistry, Vienna University of Technology, Favoritenstrasse 9, 1040 Vienna, Austriab Institute of Materials Science and Technology, Vienna University of Technology, Favoritenstrasse 9-11/E308, 1040 Vienna, Austria

c Ivoclar Vivadent AG, Bendererstrasse 2, 9494 Schaan, Liechtenstein

Available online 8 September 2014

bstract

Lithography-based additive manufacturing (AM) is increasingly becoming the technology of choice for the small series or single unit production.t the TU Vienna a digital light processing (DLP) system was developed for the fabrication of complex technical ceramics, requiring high levelsf detail and accuracy. The DLP-system used in this study creates a ceramic green part by stacking up layers of a photo-curable resin with a solidoading of around 45 vol.% zirconia. After a thermal debinding and sintering step the part turns into a dense ceramic and gains its final properties.he native resolution of the DLP process depends on the light engine’s DMD (digital mirror device) chip and the optics employed. Currently it

s possible to print 3D-structures with a spatial resolution down to 40 �m. A modification of the light source allows for the customization of theight curing strategy for each pixel of the exposed layers. This work presents methods to improve the geometrical accuracy as well as the structuralroperties of the final 3D-printed ceramic part by using the full capabilities of the light source. On the one hand, the feasibility to control theimensional overgrowth to gain resolution below the native resolution of the light engine—a sub-pixel resolution—was evaluated. Overgrowthccurs due to light scattering and was found to be sensitive to both exposure time and exposed area. On the other hand, different light curingtrategies (LCSs) and depths of cure (Cd) were used for the 3D-printing of ceramic green parts and their influence on cracks in the final ceramicas evaluated. It was concluded that softstart LCSs, as well as higher values for Cd, reduce cracks in the final ceramic. Applying these findings

ithin the 3D-printing process may be another step toward flawless and highly accurate ceramic parts.

2014 Published by Elsevier B.V.

eywords: Additive manufacturing; Ceramic; Zirconia; Photopolymerization; Digital light processing

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. Introduction

Additive manufacturing (AM) technologies allow for theesource-efficient fabrication of highly complex structures on

layer-by-layer basis directly from 3D data [1]. For engineer-ng applications, AMTs are used to produce near net-shaped

arts made of materials including unfilled polymers, metals oreramics. For the shaping of high strength oxide ceramics in par-icular, the most commonly used AM technologies are: fused

∗ Corresponding author. +43 15880130857.E-mail address: gerald.mitteramskogler@tuwien.ac.at

G. Mitteramskogler).

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ttp://dx.doi.org/10.1016/j.addma.2014.08.003214-8604/© 2014 Published by Elsevier B.V.

eposition modeling (FDM), selective laser sintering/meltingSLS/SLM), laser engineered net shaping (LENSTM), 3Drinting (3DP), direct ink writing (DIP), laminated object man-facturing (LOM), stereolithography (SLA), and digital lightrocessing (DLP) [2,3]. Table 1 briefly describes the mentionedM technologies and lists process-inherent assets and draw-acks. This review is based on recent literature and reflects theuthors’ opinions.

Using photochemical reactions triggered by light for thehaping of a part, instead of thermal energy (SLS, SLM) or ainder system (3DP), the photopolymerization-based technolo-ies (SLA, DLP) offer several benefits in cases where a higher

eature resolution and surface quality are required. A solid parts created on a layer-by-layer basis by photopolymerization of auspension of ceramic particles in a photosensitive resin. SLA

G. Mitteramskogler et al. / Additive Manufacturing 1–4 (2014) 110–118 111

Table 1Survey of AM technologies for the manufacturing of structural ceramics; legend to symbols: “+” good, “o” average, “−” poor.

Resolution Surfacequality

Buildspeed

Post-processing

Productioncosts

Mech.properties

Process description References

FDM − − O O + − Extruder and nozzle baseddeposition system with athermoplasticceramic–polymer material

[18,19]

SLM SLS + − O + − O High power laser fuses apolymer–ceramic compositeor directly sinters a ceramicpowder

[20,21]

LENSTM − − O O O − Ceramic powder directlyinjected into molten poolcreated by high power laser

[22]

3DP O O O − + O 3D-jetting of organic blinderon a ceramic powder bed

[19]

DIP + + O O O + Direct ink-jet printing of aceramic containing ink

[23,24]

LOM O O + O + − Placing of layers of a laser cutceramic green tape

[25,26]

SL DLP + + O − O + Photocuring of ceramic [6,27]

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ses a laser scanner to cure thin lines of slurry and draw the cross-ection of the part, whereas the DLP-system cures the wholeayer at once using a light mask, which is dynamically createdy a digital micromirror device. The state of the art for DLP sys-ems are constant parameter light curing strategies (LCS), thatllow the operator to choose the light intensity (mW/cm2) andhe exposure time (s) depending on the polymerization charac-eristics of the slurry. After one layer is completed, a fresh layerf slurry is coated and the process is repeated until the so-calledreen part is built. Different to SLS, where the ceramic parti-les are shaped and sintered in one step, a green part built withL requires a thermal treatment—including a binder burnoutdebinding) and sintering step—to achieve a dense ceramic partFigs. 1 and 2).

Debinding rates need to be carefully adjusted to the volumef the green part in order to avoid cracks in the final structure.esides a temperature treatment that is too fast, the fabricationarameters of the green part could influence cracks in the finaltructure. Bae and Halloran [4] showed in a study the influence of

abrication parameters of a green part made by SLA on cracks inhe final ceramic. They conclude that residual monomer in gapsn the light-curing pattern of the laser beam leads to cracks in

ig. 1. Parts made of Al2O3 fabricated by using the DLP-based 3D printingechnology presented in this paper.

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containing resin by a laserlight source or a LED

he structure during thermal debinding. Rather than on flawlessebinding and sintering of green parts, most literature dealingith ceramic SLA or DLP-systems focuses on the measurement

nd calculation of the photopolymerization properties of slurryompositions [5–7].

In the present work, we are the first to use a DLP-processescribed by Rohner et al. [8] to test the influence of light cur-ng parameters on cracks after debinding. On the one hand wevaluate the influence of depth of cure (Cd)—the polymerizedhickness of the slurry when light curing of a layer—on cracks.

On the other hand, we test different light curing strategiesLCS) used to minimize shrinkage strains from photopoly-erization on their influence of cracks after debinding. LCS

o minimize shrinkage strains caused by photopolymerizationre known in the field of dental composites, exhibiting sim-lar polymerization kinetics as ceramic slurries used for SL.hrinkage strains originate from the conversion of C C doubleonds, where the larger van der Waals inter-molecular spacings replaced by the smaller intramolecular covalent bonds [9].iterature on dental composites explains shrinkage strains to bene limiting factor for the performance of dental restorations10–12]. One way to reduce the inherent shrinkage is by usingoftstart LCS [13–15]. Dewaele et al. [16] explain the reactioninetics of a softstart polymerization. A lower initial intensityreates fewer polymerization centers and a more linear, less brit-le polymer network. By delaying the gel-point before final highntensity curing, the material is given the ability to accommo-ate viscous flow before the rigid network is created and thusowering shrinkage strains.

The utilized DLP-system projects images used for light cur-ng of a layer at a native resolution of 40 �m. In this work

e show that it is possible to improve the geometrical reso-

ution of the green part without changing the projection optics,educing the overall building size of the 3D printer. In turbid

112 G. Mitteramskogler et al. / Additive Manufacturing 1–4 (2014) 110–118

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ig. 2. Green parts (a), sintered parts (b), and a cellular structure [28] made of

lurries, the light-cured geometry grows beyond the intentionallyxposed image by scatter light (overgrowth) that originates at theesin–particle interface due to the materials’ different refractivendexes [17]. We show that an exact control of the overgrowthllows for a green part resolution below the native resolution ofhe DLP-system, defined as the sub-pixel resolution.

. Materials and methods

.1. Ceramic DLP-system and customized LCS

The ceramic DLP-system (Fig. 3) is equipped with a specialoating device—consisting of a rotating mechanism and a coat-ng blade—enabling the processing of highly viscous ceramiclurries. The coating device provides a fresh layer of slurry andhe DLP-system selectively exposes the slurry to light at 460 nmavelength to initiate a radical photopolymerization. The DLP-

ystem adds the layers to fabricate the green part and typical layerhicknesses range from 15 �m to 100 �m. A digital micromir-or device (DMD) dynamically generates the images used for

Fig. 3. Principle of the ceramic DLP-system by Gruber et al. (2011).

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(c) fabricated by using the DLP-based technology presented in this paper.

ight curing. The micromirrors reflect the light either toward theaterial vat (on) or away from it onto a light absorber (off) to

reate light or dark pixels. The DLP-system allows for the fab-ication of green parts at an x/y resolution of 40 �m. The size ofhe building platform is 76.8 mm × 43.2 mm. A binary bitmapransfers the geometrical information of a layer to the DLP-ystem and a sequence file determines the order and timing inhich the image is to be displayed and hence enables grayscalesf the image. When the micromirrors of the DMD are switchedn more frequently than off, the image appears to be brighterhigher intensity). When the micromirrors are switched off morerequently than on, the image appears to be darker (lower inten-ity). With a combination of multiple images and a sequencele, grayscales for every single pixel of the exposed layer coulde created. The creation of the images and the sequence fileas performed in MATLAB (R2011b, The Mathworks Inc.,A). A standard pulse width modulation (PWM) algorithm was

sed to convert an analog light intensity curve to a digital (on,ff) signal for the light engine. Sampling rates for the PWManged from 20 Hz to 100 Hz, depending on the length of theCS.

.2. Ceramic filled photocurable slurry

A photo-reactive suspension (slurry) was prepared basedn commercially available di- and monofunctional methacry-ates. The slurry was a blend of 0.05 wt% of a highly reactivehotoinitiator, 2 wt% of a dispersant, an absorber, and 10 wt%f a non-reactive diluent. A solid loading of zirconia powderTZ-3YS-E, Tosoh, Japan) up to 45 vol% was achieved. A homo-eneous mixture was prepared by using a SpeedMixerTM DAC50 FVZ (Hauschild, Hamn, Germany). The SpeedMixerTM isased on the double rotation of the mixing cup (“dual asym-

etric centrifuge”). Both the organic and the ceramic powderere added in the cup and the mixing parameters were 2 min at500 rpm. The viscosity of the ceramic slurry was in the rangef 15 Pa s.

G. Mitteramskogler et al. / Additive Ma

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ig. 4. Overlays of the exposed test patterns (marked pixels) and the light curedamples for the outside case (a) and the inside case (b).

.3. Measuring geometrical overgrowth

Light scattering effects cause the curing of a larger area thanriginally exposed by the DLP-system. The overgrowth wasefined as the originally exposed length subtracted from thectually measured length. Fig. 4 shows overlays of the origi-ally exposed images and the cured and cleaned samples for theutside case (a) and the inside case (b). For the experiments,he light intensity of the exposed pixels was kept constant at3 mW/cm2 and the influence of exposure time (0.8–3.2 s) andotally exposed area (0.16–3.96 mm side length) on the amountf overgrowth was evaluated. For the inside case, the openingide length was kept constant at 0.36 mm. After exposure of thelurry the cured samples were cleaned and the respective dimen-ions were measured under a light microscope. The images werevaluated using an image analysis software (ImageJ, Nationalnstitutes of Health, USA).

.4. Measuring depth of cure (Cd)

We measured Cd for the given slurry at light intensities upo 13 mW/cm2 and exposure times ranging from 0.5 s to 12 s.he slurry was placed in a tray on top of the material vat at

filling height of 1 mm and leveled with a blade. Circles of0 mm in diameter were exposed and subsequently cleaned byemoving the excess slurry. Cd was measured with a micrometer

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ig. 5. Exponential (exp(x) 1, exp(x) 2) and soft start (softstart 1, softstart 2) LCS co

nufacturing 1–4 (2014) 110–118 113

auge. To further investigate the influence of oxygen inhibi-ion on Cd, the slurry was degassed under vacuum prior to thenvestigation.

.5. Influence of LCS and Cd on cracks after thermalreatment

As a testing geometry, cylindrical green parts of 10 mm iniameter and height were fabricated using a layer thicknesses of5 �m. The reference parts were fabricated using a constant LCSt 13 mW/cm2 light intensity and 1.5 s exposure time, yielding

measured Cd of around 100 �m. To evaluate the influence ofCS on cracks after thermal treatment, we used constant (ref-rence), exponential (exp 1, exp 2), and a softstart (softstart 1,oftstart 2) LCS to fabricate green parts (Fig. 5). To evaluatehe influence of Cd on cracks after thermal treatment, we fabri-ated green parts with the mentioned LCS at a Cd of 75 �m and50 �m. To achieve different values for Cd the high-intensityxposure times were varied accordingly. At least two cylindersere fabricated for each group and qualitatively compared with

he reference.

.6. Thermal analysis

After fabrication, the dimensional change of the green partsith respect to an increase in temperature was analyzed using

hermomechanical analysis (TMA), 2940 Fa. TA-Instruments.he samples were heated up from room temperature to 400 ◦Cith a heating rate of 0.1 K/min. We evaluated the dimensional

hange for the LCS (softstart and constant) and the differentalues for Cd (75 �m and 150 �m) we used to fabricate thereen parts.

.7. Thermal treatment

Fig. 6 shows the thermal treatment used in this study for

rying, debinding and the final sintering of the parts. Prior to thistudy, TMA measurements were used to adjust the temperaturerofile to the geometry of the green part.

mpared to constant curing (reference). 100% light intensity equals 13 mW/cm2.

114 G. Mitteramskogler et al. / Additive Manufacturing 1–4 (2014) 110–118

rying

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. Results and discussion

.1. Geometrical overgrowth

Figs. 7 and 8 show the measured overgrowth for the out-ide and the inside case calculated by subtracting the originallyxposed length by the actually measured length. At a givenight intensity of 13 mW/cm2 the overgrowth increases with bothxposure time and exposed area. For the outside case, light scat-ering causes the actual length to be higher than the originalength. For the inside case, it causes the actual length to bemaller than the original length or even leads to a completelosure of the opening. A peak could be observed for both theutside and inside cases at 2 mm and at 4 mm side length, respec-ively. The higher amount of overgrowth for the inside case coulde explained by the larger amount of surface surrounding thenner opening.

.2. Depth of cure (Cd)

For the given slurry, an increase in exposure time causes anncrease in Cd, which is converging to a maximum value depend-ng on the light intensity applied (Fig. 9). For the reference curing

tdlc

Fig. 7. Overgrowth of the outside length as a function

, debinding, and sintering).

arameters of 13 mW/cm2 light intensity and 1.5 s exposureime, a Cd of around 100 �m could be measured. The iso-energyines show that Cd is not linear with respect to the overall energypplied but higher intensities are favored for increased Cd. Tovaluate the influence of oxygen inhibition, Cd was measuredor a degassed slurry and about three times greater values for Cdould be observed (Fig. 10).

.3. Influence of LCS and Cd on cracks after debinding

Due to a certain level of translucency of the dense zirconia,racks within the sintered ceramic can be visualized by ray-ng with visible light. The side view (a) and top view (b) of

sintered reference specimen in front of a light source showrominent horizontal and vertical cracks (Fig. 11). The hori-ontal cracks do not entirely follow the layerwise orientationnd therefore might not originate from delamination issues dur-ng 3D-printing. The horizontal crack patterns instead show

stepwise crack propagation between the layers. Due to sin-

ering of the loosely packed powder to a compact ceramic, aimensional shrinkage of around 20% occurred. Fig. 12 showsight microscopy images of the rayed ceramic cylinders fabri-ated with the LCS presented above—yielding a Cd of 100 �m.

of rectangle side length and exposure duration.

G. Mitteramskogler et al. / Additive Manufacturing 1–4 (2014) 110–118 115

Fig. 8. Overgrowth of the inside length as a function of side length and exposure duration.

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ig. 9. Measured Cd for different light curing parameters. The solid lines repreevel of energy dose (iso-energy).

sing the shorter exponential LCS exp(x) 1 showed no clear

eduction of cracks compared with the reference constant LCSFig. 12(a)). Longer times for the initial intensity increaseexp(x) 2) prevented vertical cracks and led to only minor

sc(

Fig. 10. Cd for degassed (dashed lines

e results for the same intensity (iso-intensity); the dashed lines show the same

orizontal cracks (Fig. 12(b)). Cylinders fabricated with the soft-

tart LCS showed no vertical cracks and only smaller horizontalracks. The fewest cracks were observed for the LCS softstart 2Fig. 12(d)).

) and regular (solid lines) slurry.

116 G. Mitteramskogler et al. / Additive Manufacturing 1–4 (2014) 110–118

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ig. 11. Side view (a) and top view (b) of a sintered reference cylinder built wxposure duration; lines mark the cracks visible within the ceramic.

To evaluate the influence of Cd on cracks after debinding, wearied the high intensity curing time from originally 1.5 s to 0.8 snd 2.5 s, yielding values for Cd of 75 �m and 150 �m respec-ively. At the same layer thickness of 25 �m, cylinders fabricatedt a Cd of 75 �m again showed structural cracks similar to theeference part shown above. Using a Cd of 150 �m and the con-

tant LCS it was not possible to achieve flawless ceramics. Theombination of the softstart LCS and a Cd of 150 �m resultedn flawless specimens as seen in Fig. 13.

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ig. 12. Sintered specimens fabricated with exponential LCS exp(x) 1 (a) and expropagation are highlighted.

constant light intensity curing protocol at 13 mW/cm2 light intensity and 1.5 s

The curing of layer with the presented LCS softstart 2 can beegarded as a double light exposure approach. The initial softerolymerization at 1.3 mW/cm2 for 10 s allows for a certain levelf viscous flow of the slurry before the gel-point is reached.

further reduction of shrinkage stresses might be achieved bysing even lower softstart intensities. As a drawback, the curing

f the layers turns into a time consuming process, since loweright intensities require longer light exposure times for similaruring results. The high-intensity light exposure at 13 mW/cm2

(x) 2 (b) and softstart LCS softstart 1 (c) and softstart 2 (d). Lines of crack

G. Mitteramskogler et al. / Additive Ma

Fig. 13. Flawless cylindrical part fabricated with the LCS softstart 2 and a Cd

of 150 �m.

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icgytwteioincreased shrinkage compared with the specimens fabricatedwith the constant LCS, see Fig. 15. The temperature at which a

or 2.5 s completes the curing of the slurry and ensures the bond-ng to the previous layer. We have concluded that a higher depthf cure increases the bonding of the layers and improves the

tructural properties of the final ceramic part. m

Fig. 14. SEM micrographs of fracture surfaces of green p

Fig. 15. Result of TMA measurements. The samples were

nufacturing 1–4 (2014) 110–118 117

.4. Influence LCS and Cd on structural and thermalroperties

We analyzed the fracture surface of green parts fabricated at Cd of 75 �m and 150 �m using scanning electron microscopySEM) imaging. At a Cd of 75 �m the fracture surface showsigns of the layered manufacturing route (Fig. 14(a)). At a Cdf 150 �m the green structure seems homogeneous (Fig. 14(b)),hich might indicate improved interlaminar bonding of the sin-le layers.

Thermomechanical analysis (TMA) of green parts with 4 mmn diameter and height revealed a difference in dimensionalhange over temperature. The dimensional shrinkage of thereen part is associated with evaporation of the diluent and pyrol-sis of the organic. The escaping gaseous products pass throughhe mesh of ceramic particles and the remaining polymer net-ork. At higher temperature rates, trapped debinding gasses lead

o an increase in internal pressure of the green part visible as anxpansion in the TMA measurement. Using TMA, cracks dur-ng debinding of the green part can be seen as a peak in the ratef shrinkage. Specimens fabricated with a softstart LCS showed

ajor mass loss begins shifted to a higher temperature for the

arts fabricated at a Cd of 75 �m (a) and 150 �m (b).

heated up to 400 ◦C at a heating rate of 0.1 K/min.

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pecimens at 150 �m. This might be explained by a more rigidetwork caused by longer exposure times, which increase thectivation energy needed for the evaporation of the diluent. Theigher amount of total shrinkage for parts fabricated with theoftstart LCS may be explained by a more rigid network afterhotopolymerization, which causes higher levels of shrinkageuring the drying of the part.

. Conclusion

This work presents a modification of a special DLP-systemsed for the 3D-printing of ceramic green parts in order tomprove the geometrical accuracy, as well as the structural prop-rties of the final structural ceramic part after debinding andintering. We found that light scattering within the ceramic filledlurry causes a certain amount of widening of dimensions inhe final geometry and that this overgrowth is both sensitiveegarding overall exposure area and exposure time. We con-luded that softstart LCS and higher values for Cd are preferredor the successful thermal processing of a 3D-printed green part.oftstart LCS are applied to reduce internal stresses originat-

ng from photopolymerization, whereas higher values for Cdmprove the interlaminar bonding of the layers.

To implement these findings in the DLP-system, the imagesed for light curing of a layer could be split into a core image and

contour image, which are light cured consecutively. At a certainidth of the contour the amount of overgrowth could be con-

rolled by the light exposure duration. This approach achieves resolution below the spatial resolution of the DLP-system,efined as sub-pixel resolution, without a loss in overall buildingize. Besides the time-controlled curing of the contour images,he core image of a layer could be light-cured with the presentedoftstart LCS to improve the structural properties of the finaleramic part.

cknowledgments

The research presented in this paper was conducted at theechnical University of Vienna. We gratefully acknowledge thenancial support from the Christian Doppler Laboratory forhotopolymers in Digital and Restorative Dentistry and Ivoclarivadent AG (Liechtenstein).

ppendix A. Supplementary data

Supplementary data associated with this article can be found,n the online version, at doi:10.1016/j.addma.2014.08.003.

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